Process for the prevention of tar still corrosion



PROCESS FOR THE PREVENTION OF TAR STILL CORROSION United States Patent Company, Inc-., a corporation of Delaware No Drawing. Application May 7, i957 Serial No. 657,513 r 3 Claims. (Cl. 208--34S) This invention relates generally to a process for the distillation of coal tar fiom the low temperature carbonization of coal and more particularly to a process and composition for preventing the corrosion of the equipment used in the distillation of such coal tar.

The composition of the tar produced by the carbonization of coal depends upon the temperature of carbonization of the coal, and the coal carbonization may roughly be considered as a high temperature or a low temperature process. The coke produced by the high temperature process is usually for blast furnace use, and the coke produced by the low temperature process is usually for domestic or boiler use.

Tar is the most important by-product from low temperature carbonization of coal. The tars from low temperature carbonization are usually dark brown liquids which are quite fluid at ordinary temperatures and are less viscous than the tars from high temperature carbonization. The compositions of low temperature tars vary more widely than do the compositions of the high temperature tars because the low temperature tars have not been subjected to the severe secondary cracking reaction that occurs at the higher temperatures. The tar from both processes is usually refined by distillation.

It has been found that corrosion of the equipment used .in the distillation of low temperature tar is particularly great when the equipment is of the carbon steel type. An article entitled The Development of Liquid Products From Low Temperature Carbonization by W. A. Bristow, Journal of the Institute of Fuel, April 1947, for example, reports that in a large British plant for the distillation of low temperature tar, mild steel stills had a throughput life of only 1% to 2 million gallons of crude tar, and that after extensive research, it was necessary to employ equipment of special alloys and stainless steel to keep the plant in operation. This solution to the problem of corrosion encountered in the distillation of low and mid temperature tars, although costly, has been the only solution heretofore found practical.

High temperature tars can also be corrosive to the equipment used for their distillation. The corrosiveness of such tars, however, is usually attributed to ammonium chloride which is commonly present in such tar and which is believed to dissociate during the process of distillation, thereby forming hydrochloric acid. The corrosion rate of such high temperature coke oven tar has been controlled either (1) by prior washing of the tar with water to remove the soluble ammonium chloride or (2) by the addition of alkaline reagents, such as lime or caustic soda, which by metathesis form chlorides, as calcium or sodium chlorides, that do not dissociate to form hydrochloric acid.

The foregoing treatment for reducing the corrosiveness of high temperature tars, however, has been found not applicable to low temperature or mid temperature tars.

. For example, it was found that one type of equipment temperature tar actually has a life of less. than three months when used for the distillation of low temperature tars. The addition of lime, as in the case of high temperature tar, extended the life of the equipment to approximately nine months. Thus, although such addition of an alkaline reagent reduced the rate of corrosion, the rate was still extremely high.

An object of the present invention, therefore, isto provide a novel process for the distillation of low temperature tars wherein the corrosive action of the tars is reduced.

Another object of the invention is to provide a novel process for the distillation of low temperature tars in mild steel equipment without extreme corrosion of the equipment.

A further object of the invention is to provide a novel low temperature tar composition that will not corrode conventional steel distillation equipment.

It is believed that the low temperature tars such as lignite tars, tars made by the well known Disco process, or mid temperature tars produced from vertical retorts owe their corrosiveness to the presence of carboxylic acids. These low temperature tars contain carboxylic acids such as acetic, butyric and benzoic acid, to the extent of 0.4 to 2 percent. A basis for this belief arises from the fact that the corrosion rate of Disco tar has been found to be somewhat temperature dependent: the liquid corrosion rate of wet Disco tar, for example, has been found to be 84 mils (thousands of an inch) per year; the corrosion rate of dry Disco tar to be 650 mils per year; and the corrosion rate of Disco tar after topping of 10 percent oil to rise 1100 mils or greater than one inch per year. In the foregoing conditions, the temperature in the still is rising; but after-the 10 percent distillation point has been reached, the corrosion rate has been found gradually to decrease. It is believed that the reason for this decrease is that the carboxylic the tar, the corrosion rate has been found to be reduced to approximately /2 inch or 500 mils per year.

It has now been found that the presence of trace amounts of phosphoric acid inhibit corrosion during the distillation of low and mid temperature tars containing carboxylic acids. Such trace amounts of phosphoric acid, however, do not appreciably change the characteristics, that is the color or composition of the distillate obtained; and substantially none of the phosphoric acid appears in the distillate as overhead product. When such trace amounts of phosphoric acid are added to the distillation equipment that the concentration of phosphoric acid is in the range of 0.05 to 1 percent, these trace amounts effectively inhibit the corrosion of the equipment by the coal tars that contain carboxylic acids. This result was surprising because orthophosphoric acid is, per se, so corrosive to carbon steel that if the acid is to be held in mild steel containers, an inhibitor must be added to prevent corrosion of the container by the acid. It has been found that as little as 0.3 percent of phosphoric acid in the tar is sufiicient to reduce the corrosion rate tenfold. It was found that in the foregoing case where the normal corrosion rate was from 1100 mils per year, the addition of trace amounts of phosphoric acid in accordance with the present invention reduced the rate to mils; and at the 45 percent off point, the addition of trace amounts of phosphoric acid reduced the corrosion rate substantially to 0. Also in the type of equipment discussed above wherein an expected life for the distillation of tar had reduced the life to less than three months, it was found that the addition of trace amounts of phosphoric acid permitted the equipment to be operated indefinitely.

The phosphoric acid added with the input tar to the still stays in the still and is removed with the bottoms product. To prevent corrosion of the plates of the tower, trace amounts of phosphoric acid may be added to the reflux for the tower. These trace amounts then gradually descend the tower as overflow from plate to plate occurs. I I The term phosphoric acid. has been used generically herein to include ortho and meta phosphoric acid; Ortho phosphoric acid is convenient to obtain and use, and meta phosphoric acid is just as satisfactory for use in the present invention. a j

The foregoing has presented a novel process forsubstantially reducing the corrosion of equipment in which low temperature tars are being distilled by the addition of trace amounts of phosphoric'acid.

e What is claimed is: v 1

1. A process for substantially reducing the corrosion of the equipment in which low temperature tar is being distilled which comprises mixing phosphoric acid with said tar to the extent of 0.05 percent to 1 percent phosphoric acid.

2. A mixture substantially non-corrosive to mild steel reactors comprised of tar from the low temperature carbonization of coal tar and 0.05 percent to 1 percent of phosphoric acid.

3. A process for substantially reducing the corrosion in distillation towers in whichlow temperature tar is being distilled which comprises adding 0.05 to 1 percent of phosphoric acid to the reflux for the tower.

References Cited in the file of this patent UNITED STATES PATENTS 2,202,128 Towne et a1 May 28, 1940 2,393,154 H Franklin Jan. 15,1946 

1. A PROCESS FOR SUBSTANTIALLY REDUCING THE CORROSION OF THE EQUIPMENT IN WHICH LOW TEMPERATURE TAR IS BEING DISTILLED WHICH COMPRISES MIXING PHOSPHORIC ACID WITH SAID TAR TO THE EXTENT OF 0.05 PERCENT TO 1 PERCENT PHOSPHORIC ACID. 